Cities should close schools for up to three months in the event of a severe flu outbreak, ball games and movies should be canceled and working hours staggered so subways and buses are less crowded, the federal government advised today in issuing new pandemic flu guidelines to states and cities.

Health officials acknowledged that such measures would hugely disrupt public life, but they argued that these measure would buy the time needed to produce vaccines and would save lives because flu viruses attack in waves lasting about two months.

“We have to be prepared for a Category 5 pandemic,” said Dr. Martin Cetron, director of global migration and quarantine for the Centers for Disease Control and Prevention, in releasing the guidelines. “It's not easy. The only thing that's harder is facing the consequences. That will be intolerable.”

In an innovation, the new guidelines are modeled on the five levels of hurricanes, but ranked by lethality instead of wind speed. Category 1, which assumes 90,000 Americans would die, is equivalent to a bad year for seasonal flu, Glen Nowak, a C.D.C. spokesman, said. (About 36,000 Americans die of flu in an average year.) Category 5, which assumes 1.8 million dead, is the equivalent of the 1918 Spanish flu pandemic. (That flu killed about 2 percent of those infected; the H5N1 flu now circulating in Asia has killed more than 50 percent but is not easily transmitted.)

The new guidelines also advocate having sick people and all their families even apparently healthy members stay home for 7 to 10 days.

They advise against closing state borders or airports because crucial deliveries, including food, would stop. They did not offer guidance on wearing masks, but Dr. Cetron said the C.D.C. would issue advice on this soon.

The guidelines are only advisory, since authority for measures like closing schools rests with state and city officials; but many local officials had asked for guidance, Dr. Cetron said.

The federal government has taken primary responsibility for developing and stockpiling vaccines and antiviral drugs, as well as masks and some other supplies.

Today's guidelines are partly based on a recent study of how 44 cities fared in the 1918 epidemic conducted jointly by the C.D.C. and the University of Michigan's medical school. Historians and epidemiologists pored over hospital records and newspaper clippings, trying to determine what factors partly spared some cities and doomed others.

While a few tiny towns escaped the epidemic entirely by cutting off all contact with outside, most cities took less drastic measures. These included isolating the sick and quarantining homes and rooming houses, closing schools, churches, bars and other gathering places, canceling parades, ball games, theaters and other public events, staggering factory hours, barring door-to-door sales, discouraging the use of public transport and encouraging the use of face masks.

The most effective measure seemed to be moving early and quickly. For example, said Dr. Howard Markel, a medical historian and one of the study's leaders, Philadelphia, the worst-hit city, had nearly three times as many sick and dead per capita as St. Louis, which had was hit weeks later by the virus moving inland from the Eastern Seaboard and had time to react as soon as flu cases rose above averages.

“No matter how you set up the model,” Dr. Markel said, “the cities that acted earlier and with more layered protective measures fared better.”

Any pandemic is expected to move faster than a new vaccine can be produced; current experimental vaccines against H5N1 avian flu are in short supply and based on strains isolated in 2004 or 2005. Although the government is creating a $4 billion stockpile of the antiviral drug Tamiflu, it is only useful when taken within the first 48 hours, and Tamiflu-resistant flu strains have already been found in Vietnam and Egypt.

“No one's arguing that by closing all the schools, you're going to prevent the spread,” Dr. Markel added. “But if you can cut cases by 10 or 20 or 30 percent and it's your family that's spared, that's a big deal.”

School closures can be very controversial, and picking the right moment is hard, because it must be done before cases soar.

Jeffrey Levi, executive director of the Trust for America's Health, a health policy organization, noted that in poor city neighborhoods, 30 to 60 percent of all children get breakfasts or lunches crucial to their nutrition at school.

“What are you going to do about that?” he asked.

Dr. Markel said it might be possible to keep the cafeterias open and transport food to points where parents could pick it up, a move that would also keep cafeteria workers and bus drivers employed.

Several public health experts praised the C.D.C. guidelines, although there were some quibbles with aspects of them.

Dr. Michael T. Osterholm, director of the University of Minnesota's Center for Infectious Disease Research and Policy, said he saw no point in fretting over exactly when to close schools, because his experience in meningitis outbreaks convinced him that terrified parents would keep their children at home anyway.

“I don't think we'll have to pull that trigger,” he said. “The hard part is going to be unpulling it. How do the principals know when schools should open again?”

Other experts pointed out that children out of school often behave in ways that are nearly as contagious. Youngsters are sent to day care centers, and teenagers gather in malls or at each others' houses.

“We'll be facing the same problem, but without the teaching,” said Dr. Irwin Redlener, director of the National Center for Disaster Preparedness at Columbia University's Mailman School of Public Health. “They might as well be in class.”

Also, he noted, many employed people cannot afford to stay at home and the financial stress from not working could increase domestic violence. And he said most states and cities lacked the money to carry out the suggested guidelines or to stage drills of them.

Dr. Cetron argued that caring for children in groups of six or fewer cut transmission risks. He also argued that parents would keep many children from gathering.

“My kids aren't going to be going to the mall,” he said.

The historian John Barry, author of “The Great Influenza,” a history of the 1918 flu, questioned an idea underpinning the study's conclusions. There is evidence, he said, that some cities with low sickness and death rates in 1918, including St. Louis and Cincinnati, were hit by a milder spring wave of the virus. That would have, in effect, inoculated their citizens against the more severe fall wave and might have been more important than their public health measures.

The guidelines did not suggest using the military to enforce quarantines, as President Bush said he might do when he first mentioned avian flu in 2005.

Dr. Levi said that using the National Guard to set up temporary clinics or move pharmaceutical supplies might make sense.

“But they're not there,” he said. “The people who know how to run field hospitals are in Iraq.”

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